DESCRIPTION (Applicant's Description) This proposal describes a gene therapy approach for the treatment of Fanconi anemia. Fanconi anemia is typically characterized by bone marrow failure, developmental abnormalities and cancer predisposition. Recent studies have documented that 40-50% of all FA patients develop acute leukemia and solid malignancies. Our hypothesis will test whether gene correction of hematopoietic stem/progenitor cells will lead to a competitive growth advantage of the gene-corrected cells and would correct the predisposition to bone marrow failure and leukemic transformation. Current therapy for FA patients is allogeneic bone marrow transplantation from a histocompatible donor. However, most patients lack a suitable donor and usually die from aplasia or acute leukemia. Thus, alternative therapies must be investigated. The biochemical defect of FA is not yet known but FA cells display characteristics which indicate a defect in DNA repair. The hallmark of the FA disease is the hypersensitivity of FA cells to DNA cross linking agents such as mitomycin C. To date, five different complementation subtypes of FA have been identified from somatic cell hybridization studies. Of these, the FA-A group is the most prevalent, comprising up to 70% of FA cases. Recently, the gene responsible for FA-A was identified and cloned. A retroviral vector carrying the FAA cDNA was constructed as well as a packaging cell line to produce the vector. The retroviral vector, FAA5.5 clone 17, was tested for its ability to transduce and phenotypically correct cells obtained from FA patients. Retroviral-mediated transduction of CD34-immunoselected hematopoietic cells from four different FAA patients resulted in phenotypic correction. Expression of the FAA transgene was adequate to normalize cell growth in the presence of mitomycin C. These experiments were an early indication of the feasibility of treating FA patients through transfer of the FAA gene into hematopoietic progenitor cells. The gene therapy protocol for the treatment of FA involves using a retroviral vector carrying the FAA gene to transduce hematopoietic progenitor cells obtained from an FA-A donor patient. Following transduction, cells will be reinfused into the donor patient. The objectives of the clinical protocol are (1) to establish the safety and feasibility of retroviral-mediated transfer of FAA into hematopoietic D progenitor cells, (2) determine the extent of engraftment following reinfusion of the transduced hematopoietic progenitors without prior ablation of recipient marrow, and (3) determine if FA patients receiving FAA-transduced cells will exhibit improved hematopoietic function.